1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, this invention relates to interelectrode separator systems useful in electrochemical cells.
2. Description of Related Art
Electrochemical cells utilizing consumable, reactive metal anodes are well known. Typically, the anode comprises an alkali metal, such as lithium, in elemental, compound or complex form, in conjunction with a cathode and an aqueous alkaline electrolyte. In a preferred form, the anode is lithium, the cathode is an active, porous material such as silver oxide, and the electrolyte comprises an aqueous solution of lithium hydroxide. Such cells are described in detail in Rowley U.S. Pat. No. 3,791,871 issued Feb. 12, 1974 and in Momyer et al, U.S. Pat. No. 4,269,907 issued May 26, 1981, the respective disclosures of which are incorporated herein by reference.
Electrochemical cells utilizing a nonconductive flow screen to separate a reactive metal anode from a cathode are well known. Typically, a porous, expandable element is positioned between the flow screen and the cathode to maintain essentially uniform dimensions for the electrolyte flow channel defined between the electrodes as the anode is consumed during cell operation. This element is of a thickness and porosity such that it can be compressed and yet allow the electrolyte to pass through it freely. As the anode is consumed during operation, the porous element expands from its initial compressed state against the flow screen which is positioned against and between the anode and the porous element, thereby maintaining the flow screen in position against the anode at all times during cell operation. Such a cell is described in detail in Momyer et al, U.S. Pat. No. 4,269,907.
Shock contact between the anode and cathode can cause a highly exothermic spontaneous reaction. Separator systems such as that described in Momyer et al serve to keep the active electrode materials isolated from one another. The rigid plastic screen, typically of polypropylene, provides rigid separation while the porous, expandable element, such as a compressible reticulated foam, adds to the shock and vibration resistance. Such a foam also may promote turbulent flow of electrolyte between the electrodes. Turbulent uniform flow reduces cell polarization and allows discharge at high power levels.
All previous studies have been conducted with a rigid porous separator member disposed adjacent and in direct contact with the reactive metal anode. Laying a heavy screen directly against a lithium surface can, however, cause voltage loss due to polarization, with consequently increased heat production, both of which are undesirable.